1. Field of the Invention
The present invention relates to an organic light emitting display device and a method of driving the same.
2. Description of Related Art
Recently, various types of flat panel display devices having reduced weight and volume in comparison to cathode ray tubes have been developed. Flat panel display devices include liquid crystal display devices, field emission display devices, plasma display panels, and organic light emitting display devices, among others.
Among these flat panel display devices, the organic light emitting display device displays images using organic light emitting diodes that emit light through the recombination of electrons and holes. The organic light emitting display device has a fast response time and is driven with low power consumption.
FIG. 1 is a circuit diagram of a conventional pixel of an organic light emitting display device. In FIG. 1, transistors included in the pixel are NMOS transistors.
Referring to FIG. 1, the conventional pixel 4 of the organic light emitting display device includes an organic light emitting diode OLED and a pixel circuit 2 connected to a data line Dm and a scan line Sn to control the organic light emitting diode OLED.
An anode electrode of the organic light emitting diode OLED is coupled to the pixel circuit 2, and a cathode electrode thereof is coupled to a second power source ELVSS. The organic light emitting diode OLED emits light having luminance corresponding to current supplied from the pixel circuit 2.
The pixel circuit 2 controls an amount of current supplied to the organic light emitting diode OLED corresponding to a data signal supplied to a data line Dm when a scan signal is supplied to a scan line Sn. To this end, the pixel circuit 2 includes a second transistor M2 (i.e., a driving transistor) coupled between a first power source ELVDD and the organic light emitting diode OLED; a first transistor M1 coupled between the second transistor M2 and the data line Dm, with a gate electrode coupled to the scan line Sn; and a storage capacitor Cst coupled between a gate electrode and a second electrode of the second transistor M2.
A gate electrode of the first transistor M1 is coupled to the scan line Sn, and a first electrode thereof is coupled to the data line Dm. A second electrode of the first transistor M1 is coupled to one terminal of the storage capacitor Cst. Here, the first electrode is either a source or a drain electrode, and the second electrode is the other electrode different from the first electrode. For example, if the first electrode is a drain electrode, the second electrode is a source electrode. When a scan signal is supplied to the first transistor M1 from the scan line Sn, the first transistor M1 is turned on, and a data signal supplied from the data line Dm is supplied to the storage capacitor Cst. At this time, a voltage corresponding to the data signal is charged into the storage capacitor Cst.
The gate electrode of the second transistor M2 is coupled to the one terminal of the storage capacitor Cst, and a first electrode thereof is coupled to the first power source ELVDD. The second electrode of the second transistor M2 is coupled to the other terminal of the storage capacitor Cst and the anode electrode of the organic light emitting diode OLED. The second transistor M2 controls an amount of current flowing from the first power source ELVDD through the organic light emitting diode OLED to the second power source ELVSS, the amount of current corresponding to the voltage stored in the storage capacitor Cst.
One terminal of the storage capacitor Cst is coupled to the gate electrode of the second transistor M2, and the other terminal thereof is coupled to the anode electrode of the organic light emitting diode OLED. A voltage corresponding to a data signal is charged into the storage capacitor Cst.
The conventional pixel 4 displays an image having a predetermined luminance by supplying current to the organic light emitting diode OLED corresponding to the voltage charged in the storage capacitor Cst. However, in such a conventional organic light emitting display device, images having uniform luminance are very difficult to display due to the threshold voltage variation of the second transistor M2.
Threshold voltages of second transistors M2 in respective pixels 4 are different from each other, and the respective pixels 4 generate light having different luminance in response to a same data signal. Therefore, images having uniform luminance cannot be displayed.